1. Technical Field
The present invention relates to a spindle motor, more particularly the spindle motor suitable for driving a hard disk or disks of the hard disk drive device.
2. Description of the Prior Art
In the case of the prior art as shown in FIG. 3, the hard disk drive motor of the hard disk drive device is provided with a shaft 22 secured on a base 21 to extend vertically therefrom, and a rotor hub 25 journalled rotatably around the shaft through a pair of upper and lower bearings 23, 24. The outer peripheral surface of the bearings 23, 24 are mounted within the vertical through bore 25a of the rotor hub. In FIG. 3, the reference numeral 26 is attached to a spacer to be disposed between the bearings, the reference numeral 27 is attached to a seal ring, the reference numeral 28 is attached to rotor magnets, the reference numeral 29 is attached to stators, and the reference numeral 30 is attached to stator coils. The spacer and the seal ring are adapted to be secured on the inner peripheral surface of the through bore of the rotor hub.
The temperature of the spindle motor will vary under the effect of the heat produced during the operation of the motor itself, or the changing of the atmospheric temperature, however, temperature variation of upper and lower ball bearing is different. This will sometimes lead the temperature difference between the upper and lower ball bearings 23, 24. This temperature difference between bearings will also vary the size of the bearings, i.e. the bearing of higher temperature will expand beyond the size of the lower one. This variation of the size of the bearings will cause the stress to be applied to the rotor hub to deform it to reduce the accuracy of the rotation of the rotor hub.
The outer races of the ball bearings are generally made of the ferrous metal material and the rotor hub 25 of the interior surface 25a of which the outer races are adapted to be interference fitted or adhesively secured is generally made of aluminum material larger in its coefficient of expansion than that of the ferrous metal material.
When the temperature of the spindle motor is decreased from the temperature during the assembling operation of the ball bearings into the rotor hub, the outer races of the ball bearings are apt to deformed under the squeezing effect due to the thermal retraction of the rotor hub. This will cause the reduction of the accuracy of the rotation of the rotor hub or noise produced during the rotation of the rotor hub.
Further, the rotor hub of aluminum material has a tendency to be damaged or deformed by the outer races of the ferrous metal material while the ball bearings are to be incorporated into the rotor hub. This is because the aluminum material of the rotor hub is lower in its hardness than that of the outer races of the bearings. This will impair the accuracy of the assembly so that the accuracy of the rotation of the rotor hub is also decreased.
In the spindle motor of the hard disk drive device, the lowered accuracy of rotation of the rotor hub will lean the hard disk or magnetic storage medium, or drive the disk in an eccentric manner to occur the disk crush to impair the reliability of the hard disk drive device.
Accordingly, the object of the present invention is to provide a spindle motor of high accuracy of rotation irrespective of the temperature differential for driving the hard disk or disks of the hard disk drive device.